The present application relates to multi-period, thin-film structures, and to techniques for forming such structures.
Magnetoresistance refers to the dependence of the resistance of ferromagnetic materials on the relative orientation of the current and magnetization directions. There are several “flavors” of magnetoresistance, each being attributed to different underlying mechanisms. Technology developed in recent years by Integrated Magnetoelectronics (IME) of Berkeley, Calif., is based on layered magnetic structures characterized by either giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR); collectively, quantum magnetoresistance (QMR). See, for example, the various U.S. patent documents incorporated by reference below. This technology includes both magnetic memories and magnetic circuits; the latter being based on the Transpinnor®, IME's proprietary solid-state circuit component that can replace a variety of semiconductor components, including the semiconductor transistor.
Optimal performance of QMR devices calls for QMR structures with low drive fields and high values of QMR. There has been substantial activity in the development of TMR structures over the past decade, driven in large measure by theoretical predictions that TMR values of 1000% or more should be realizable in structures in which the amorphous Al2O3 barrier layer that was originally used is replaced with a polycrystalline MgO barrier layer. Several experimental groups have achieved room-temperature values up to 220%, using an MgO insulator, and TMR values around 200% are now found routinely in simple structures. More recently, TMR values over 1000% at room temperature have been observed under special conditions.
The experimental situation with regard to GMR is significantly different. Despite massive industry and academic efforts over more than two decades, the GMR value has not budged over about 20% for simple structures. Values of GMR around 100% have been achieved in a class of so-called superlattices (i.e., multi-period structures with many periods of a repeating pattern of magnetic layers separated by non-magnetic layers) that are coupled anti-ferromagnetically across the active interfaces, but this class of structures typically requires very large switching fields that are impractically large for commercially viable devices.
It may seem therefore that TMR should be the effect of choice for devices and systems based on magnetoresistance. There is, however, a compelling reason for using GMR rather than TMR. Though TMR-based devices are expected to be viable down to near nanoscale features, thermal stability of QMR devices becomes a significant issue at the deep nanoscale level, and this issue is much more readily addressed using GMR than TMR.
Resistance of the simplest GMR structure—two magnetic layers separated by a non-magnetic metal spacer such as chromium (Cr), copper (Cu), or ruthenium (Ru)—is relatively low if the two magnetizations are parallel, relatively high if anti-parallel. This is the case irrespective of whether the exchange coupling between the two layers is ferromagnetic or anti-ferromagnetic.
There is an exchange coupling between two magnetic layers through the non-magnetic spacer between them. Exchange coupling is an indirect interaction mechanism of the magnetic layers mediated by the non-magnetic spacer layer. This coupling can be either ferromagnetic or anti-ferromagnetic. If the former, the direction of magnetization (also referred to herein as the magnetization vector) of the two magnetic layers tend to be aligned or parallel in the low-energy or ground state (e.g., in the absence of an external magnetic field), i.e., the low-resistance configuration. By contrast, for GMR structures in which the exchange coupling is anti-ferromagnetic, the magnetization vectors tend to be anti-parallel in the ground state, i.e., the high-resistance configuration.
If the coupling is anti-ferromagnetic, it is possible to realize parallel alignment of the magnetization vectors, and therefore the GMR effect, by saturating the structure. Superlattices (structures having many multi-layer periods, e.g., >about 50 periods) in which the exchange coupling is anti-ferromagnetic have been shown to have large values of GMR but, if the anti-ferromagnetic coupling is very strong, the magnetic fields necessary to drive the structures to saturation are impractically large for use in commercially viable systems or devices, e.g., on the order of 10,000 Oersteds (Oe).
The current understanding of the nature of the exchange coupling in GMR structures—ferromagnetic or anti-ferromagnetic—is as an oscillatory function of spacer thickness, with the strength of the coupling decreasing with increasing thickness of the non-magnetic layers separating the magnetic layer. See, for example, FIG. 1 in which a coupling field He is shown as a function of copper spacer thickness for a [Cu/Co]×50 lattice. Positive He corresponds to spacer thicknesses which results in anti-ferromagnetic coupling. Negative He corresponds to spacer thicknesses which results in ferromagnetic coupling. FIG. 2 shows GMR (lower plot) as a function of spacer thickness for the [Co/Cu]×50 lattice of FIG. 1. As shown, for spacer thicknesses corresponding to the ferromagnetic coupling regions of the structure, no GMR effect is observed. The four small graphs in FIG. 2 show the normalized magnetization of the two magnetic layers as a function of applied magnetic field in kilo-Oersteds.